Field of the Invention
The present invention relates to pneumatic tires. More specifically, the present invention relates to pneumatic tires having narrow grooves formed in shoulders.
Description of the Related Art
FIG. 3 shows a shoulder rib 4 portion (also referred to as a shoulder portion) of a conventional tire 2. As shown in FIG. 3, the tire 2 includes a tread 6 and sidewalls 8. The sidewalls 8 extend almost inward from the tread 6 in the radial direction. In the tire 2, the sidewalls 8 are layered on the tread 6. Such a structure is also referred to as an SOT (Sidewall on Tread) structure. In the SOT structure, an end PK of each sidewall 8 is positioned on a side surface 12 that extends almost inward from an end PE of a tread surface 10 in the radial direction. A distance, (indicated by a double-headed arrow HS in FIG. 3), in the radial direction, from the end PE of the tread surface 10 to the end PK of the sidewall 8 is typically set as appropriate so as to be in a range from 3 mm to 40 mm.
The tread 6 includes a base layer 14 and a cap layer 16. The cap layer 16 is disposed outward of the base layer 14 in the radial direction, and covers the entirety of the base layer 14. For the cap layer 16, a crosslinked rubber that is excellent in wear resistance, heat resistance, and grip performance is used. The end PK of the sidewall 8 described above is also an end of a boundary 18 between the cap layer 16 and the sidewall 8. For the base layer 14, a crosslinked rubber excellent in adhesiveness is used. From the viewpoint of reduction of rolling resistance, low heat generation properties may also be considered for a crosslinked rubber for the base layer 14.
The tire 2 treads on a road surface at the tread 6. Thus, a ground-contact surface is formed between the tire 2 and the road surface. The end of the ground-contact surface is disposed in the shoulder portion of the tire 2, and a contact pressure at the shoulder portion tends to be higher than that in another portion. Therefore, a problem arises that wear is likely to develop in the shoulder portions. In particular, such wear, in other words, uneven wear is found in various portions in the tire 2 which is a heavy duty tire mounted to a truck, a bus, or the like in which heavy load acts on the tire 2.
As shown in FIG. 3, the shoulder rib 4 may auxiliarly have a narrow groove 20 formed therein. It is known that such a technique in which the groove 20 is formed in the shoulder rib 4 is effective for reducing uneven wear. Meanwhile, it is also known that damage is likely to occur at the bottom of the groove 20, and the groove 20 may not effectively function to reduce the uneven wear depending on the degree of the damage. In order to improve durability and allow the groove 20 to effectively function, various studies are performed. Examples of the studies are disclosed in JP2013-237283 and JP2001-158208.
In JP2013-237283, a technique in which a reinforcing layer including a reinforcing cord is provided along the above-described groove 20 to improve durability, is disclosed. In order to apply this technique, the reinforcing layer needs to be newly added as a component of the tire 2. In this technique, although durability can be improved, productivity is likely to be reduced.
In JP2001-158208, a technique in which the above-described groove 20 is formed so as to have a special cross-sectional shape (specifically, round-bottom-flask shape) to improve durability, is disclosed. In order to apply this technique, a mold for producing the tire 2 needs to be newly formed. Since the groove 20 has the special shape, cost may be increased not only for production of the mold but also for maintenance and management or the like. Also in this technique, although durability can be improved, productivity may be reduced.
The inventors investigate the cause of damage to the groove 20, and study the groove 20 portion in detail in order to develop a technique in which durability can be improved without reducing productivity. As a result, as shown in FIG. 3, it is found that a portion (a portion indicated by an arrow PP in FIG. 3) of the base layer 14 is formed so as to protrude and the thickness of a portion of the cap layer 16 at the bottom of the groove 20 is not sufficient. The present invention is made on the basis of the findings.
An object of the present invention is to provide a pneumatic tire that has an improved durability without reducing productivity.